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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Neuron. Author manuscript; available in PMC Jan 29, 2010.
Published in final edited form as:
PMCID: PMC2673083

Temporal Coherence in the Perceptual Organization and Cortical Representation of Auditory Scenes


Just as the visual system parses complex scenes into identifiable objects, the auditory system must organize sound elements scattered in frequency and time into coherent “streams”. Current neuro-computational theories of auditory streaming rely on tonotopic organization of the auditory system to explain the observation that sequential spectrally distant sound elements tend to form separate perceptual streams. Here, we show that spectral components that are well separated in frequency are no longer heard as separate streams if presented synchronously rather than consecutively. In contrast, responses from neurons in primary auditory cortex of ferrets show that both synchronous and asynchronous tone sequences produce comparably segregated responses along the tonotopic axis. The results argue against tonotopic separation per se as a neural correlate of stream segregation. Instead we propose a computational model of stream segregation that can account for the data by using temporal coherence as the primary criterion for predicting stream formation.


When listening to someone at a crowded cocktail party, or trying to follow the second violin line in a symphonic orchestra, we rely on our ears’ and brain’s extraordinary ability to parse complex acoustic scenes into individual auditory “objects” or “streams” (Griffiths and Warren, 2004). Just as the decomposition of a visual scene into objects is an challenging and mathematically ill-posed problem, requiring both “top-down” and “bottom-up” information to solve (Marr, 1983; Zeki, 1993), the auditory system uses a combination of acoustic cues and prior experience to analyze the auditory scene. A simple example of “auditory streaming” (Bregman, 1990; Carlyon, 2004) can be demonstrated and explored in the laboratory using sound sequences like those illustrated in Figure 1. These sequences are produced by presenting two tones of different frequencies, A and B, repeatedly (Fig. 1A). Many psychophysical studies have shown that this simple stimulus can evoke two very different percepts, depending on the frequency separation, ΔF, between the A and B tones, and the time interval, ΔT, between successive tones (for a review see Bregman, 1990). In particular, when ΔF is relatively small (< 10%), most listeners perceive and describe the stimulus as a single stream of tones alternating in frequency, like a musical trill. However, when ΔF is large, the percept is that of two parallel but separate streams, each containing only tones of the same frequency (A-A- and B-B-) – see supplementary materials for an auditory demonstration. The perceptual separation of sound components into distinct streams is usually referred to as “stream segregation”; the converse process is variously known as “stream integration”, “grouping”, or “fusion”. Manifestations of auditory streaming have been observed in various non-human species, including birds, fish, and monkeys, suggesting that streaming is a fundamental aspect of auditory perception, which plays a role in adaptation to diverse ecological environments (Bee and Micheyl, 2008; Fay, 1998, 2000; Hulse et al., 1997; Izumi, 2002; MacDougall-Shackleton et al., 1998).

Figure 1
Schematic spectrograms of stimuli used to study the perceptual formation of auditory streams. (A) The typical stimulus used in the vast majority of psychophysical and physiological studies of auditory streaming: a sequence of tones alternating between ...

Inspired by the observation that frequency-to-place mapping, or “tonotopy”, is a guiding anatomical and functional principle throughout the auditory system (Eggermont, 2001; Pickles, 1988), current models of auditory streaming rely primarily on frequency separation for sound segregation (Beauvois and Meddis, 1991, 1996; Hartmann and Johnson, 1991; McCabe and Denham, 1997). These models predict that consecutive sounds will be grouped perceptually into a single auditory stream if they activate strongly overlapping tonotopic “channels” in the auditory system. In contrast, sounds that have widely different spectra will activate weakly overlapping (or non-overlapping) channels, and be perceptually segregated (i.e., heard as separate streams). In this way, models based on tonotopic separation can account for behavioral findings that show an increase in perceived segregation with increasing frequency separation (Hartmann, 1991). By additionally taking into account neural adaptation and forward suppression of responses to consecutive tones, these models can also account for the influence of temporal stimulus parameters, such as the inter-tone interval or the time since sequence onset, on auditory streaming (Beauvois and Meddis, 1991, 1996; Bee and Klump, 2004, 2005; Fishman et al., 2004; Fishman et al., 2001; Hartmann and Johnson, 1991; Kanwal et al., 2003; McCabe and Denham, 1997; Micheyl et al., 2007b; Micheyl et al., 2005; Pressnitzer et al., 2008).

Although tonotopic separation is important, it is clearly not the only determinant of auditory perceptual organization. Another factor is the relative timing of sounds. Sounds that start and end at the same time are more likely to be perceived as a single event than sounds whose onsets and offsets are staggered by several tens or hundreds of milliseconds (Darwin and Carlyon, 1995a). Accordingly, if the AB tone pairs were presented synchronously (as in Fig 1B) instead of sequentially (as in Fig 1A), they might form a single perceptual stream, even at large frequency separations. This prediction poses a serious problem for purely tonotopic models of auditory streaming. Unfortunately, nearly all perceptual studies of auditory streaming so far have used strictly sequential, temporally non-overlapping, stimuli (Fig. 1A), although one informal description of an experiment involving partially overlapping stimuli exists (Bregman, 1990, p. 213). On the physiological side, it is unclear how synchrony affects neural responses in the primary auditory cortex (AI), where previous studies have identified potential neural correlates of auditory streaming using purely non-overlapping stimuli (Fishman et al., 2004; Fishman et al., 2001; Gutschalk et al., 2005; Kanwal et al., 2003; Micheyl et al., 2007a; Micheyl et al., 2005; Snyder et al., 2006; Wilson et al., 2007). The complexity of auditory cortical responses makes it difficult to predict how responses of single AI units will be influenced by stimulus synchrony: depending on the position of the tones relative to the unit’s best-frequency, responses might be facilitated (i.e. enhanced), inhibited (i.e. reduced), or left unchanged by the synchronous presentation of a second tone within the unit’s excitatory receptive field.

Here we use a combination of psychophysics in humans, cortical physiology in ferret, and computational modeling to address these questions. We first report psychoacoustic findings, which reveal that synchronous and non-synchronous sound sequences are perceived very differently, with synchronous tone sequences heard as a single stream, even at very large frequency separations. We then present physiological findings, which show that the synchronous and non-synchronous tone sequences evoke very similar tonotopic activation patterns in AI. Together, these findings challenge the current view that tonotopic separation in AI is necessary and sufficient for perceptual stream segregation. Finally, we describe a computational model of stream segregation that uses the temporal coherence of responses across tonotopic (or other) neural channels to predict perception, and demonstrate that this model can account for the present and other psychophysical findings. By combining simultaneous and sequential perceptual organization principles that have traditionally been studied separately, the model proposed here provides a new and more general account of auditory perceptual organization of any arbitrary sound combinations. More generally, the present findings suggest that the principle of grouping information across sensory channels based on temporal coherence may play a key role in auditory perceptual organization, just as it is proposed for visual scene analysis (Blake and Lee, 2005).

A. Psychophysics

Experimentasl results

Informal listening to sound sequences like those illustrated in Figure 1A reveals that, for relatively large frequency separations between the A and B tones (e.g., 6 semitones or more), the alternating-tone sequence (Fig. 1A) usually evokes a percept of two separate streams, each with a constant pitch (A-A- and B-B-). In contrast, the sequence of synchronous tones (Fig. 1B) evokes the percept of a single stream, even at large ΔFs. In order to confirm and quantify this subjective impression, we asked listeners to discriminate between two sequences similar to those shown in Fig. 1B, except that in one of those two sequences, the last B tone was slightly shifted temporally (forward or backward), so that it was no longer exactly synchronous with the corresponding A tone. We measured the smallest temporal shift that listeners could correctly detect 79.4% of the time. Based on earlier results indicating that listeners can detect onset shifts of as little as a few milliseconds (ms) between spectral components of complex tones (Zera and Green, 1993a, b, 1995), but cannot accurately judge the relative timing of tones that fall into separate auditory streams (Bregman and Campbell, 1971; Broadbent and Ladefoged, 1959; Formby et al., 1998; Neff et al., 1982; Roberts et al., 2002; Warren et al., 1969), we reasoned that if listeners heard the A and B tones as a single fused stream, their thresholds in the asynchrony-detection task would be relatively small (i.e., a few ms), whereas if the listeners perceived the A and B tones a two separate streams, their thresholds should be substantially larger.

The results shown in Figure 2 (filled squares) support these predictions. In the condition where all the A and B tones before the last were synchronous (with inter-tone intervals of 50 ms), thresholds were small, in the 2–4 ms range. This is true at all of the three A–B frequency separations tested, including the very large one (15 semitones, which is larger than an octave). This outcome is consistent with the hypothesis that synchrony between the A and B tones promotes the perceptual integration of these tones into a single stream, even for relatively large frequency separations.

Figure 2
Thresholds for the detection of a temporal shift imposed on the last B tone in various types of stimulus sequences. The different symbols indicate different sequence types, which are represented schematically on the right. Polygonal symbols correspond ...

In order to check that thresholds in the asynchrony-detection task provided a valid marker of the listener’s auditory percept of streaming, three control conditions were run. The first two control conditions involved stimulus sequences in which the silent gap between consecutive B tones was either shorter (30 ms) or longer (70 ms) than that between consecutive A tones (50 ms), but with the stimulus parameters chosen such that the last A and B tones in one of the two stimulus sequences presented on a given trial would be synchronous (see Fig. 2 inset). If thresholds in the asynchrony-detection task are a faithful indicator of the listener’s auditory percept, these thresholds should be larger in these control conditions than in the main condition because a) the asynchrony between the A and B tones would promote the perceptual segregation of the stimulus sequence into two separate streams, and b) stream segregation should hamper listeners’ ability to accurately compare the timing of the A and B tones in that pair. The aim of the third control condition was to measure listeners’ sensitivity to changes in the timing of the last B tone, even when the A tones were not present. We turned off the A tones, and asked listeners to decide in which of the two presented sequences of (B only) tones the last tone was temporally shifted (either backward or forward, as in the main experiment). Therefore, in this control condition, listeners had to detect which of two presented sequences of B tones contained a temporal irregularity near the end.

Considerably larger thresholds (10–20 ms) were observed in the two control experiments, where the nominal duration of the inter-tone interval in the A tone stream was different from that in the B-tone stream, being either shorter (30 ms) or longer (70 ms) [F(1,10)=7.394, p<0.001]. This outcome is consistent with the idea that asynchrony between the A and B tones promotes the perceptual segregation into two streams, and makes it difficult, if not impossible, to accurately judge the timing of events in separate streams. In fact, the thresholds measured in those conditions were not significantly different from those measured in the third control condition, in which the A tones were turned off, so that the only cue listeners could use to perform the task was to listen for an irregularity in the timing of the B tone stream. This indicates that listeners were able to use the presence of the A tones to improve performance only when the A tones were all gated synchronously with the B tones. Overall, the psychophysical results confirm that synchronous and asynchronous tone sequences produce very different percepts, with the synchronous tones being perceived as a single stream and the asynchronous tones being perceived as two streams at large frequency separations.

B. Neurophysiology

The psychophysical results raise the question of whether neural responses to sequences of synchronous and sequential tones in the central auditory system differ in a way that can account for their very different percepts. To answer this question, we performed two experiments in which we recorded the single-unit responses in AI to sequences such as those illustrated in Fig. 1 in the awake (non-behaving) ferret. In the first experiment, we explored directly the extent of segregation between the responses to the A and B tones. In the second experiment, we assessed the range of frequencies over which the tones interacted (or mutually influenced their responses).

Experiment I: Segregation between two-tone responses

This experiment examined the distribution of responses to the two tones by translating them together, relative to the best frequency (BF) of an isolated single unit in AI of awake ferrets in five steps (labeled 1–5 in Fig. 3A), where positions 1 and 5 correspond to one of the two tones being at BF of the unit. The frequency separation (ΔF) between the tones in each test was fixed at 1, 0.5, or 0.25 octaves, corresponding to 12, 6, and 3 semitones, respectively. As described above, alternating tone sequences are usually perceived as two streams at separations of 12 and 6 semitones (1 or 0.5 octaves), but are only marginally segregated at a separation of 3 semitones (0.25 octaves). In contrast, synchronous tone sequences are always heard as one stream (Fig. 2). Therefore, if the “spatial segregation” hypothesis were valid, alternating sequences should evoke well-segregated neural responses to the far-apart tones (1 and 0.5 octaves), whereas synchronous sequences should evoke spatially overlapping responses in all cases.

Figure 3
Schematic of the tone frequencies and conditions used in the physiological experiments. Both alternating and synchronous tone sequences were tested in all conditions. (A) Experiment I: The two-tone frequencies we held fixed at one of three intervals apart ...

The results from a population of 122 units in the AI of 4 ferrets are shown in Fig. 4. In Fig. 4A, the average rate profiles for the synchronous, overlapping, and alternating presentation modes are constructed from the responses as described in the Methods. All 122 units were tested with the synchronous and alternating modes; 75/122 units were also tested with the overlapping sequences. When the tones are far apart (ΔF = 1 octave; right panel of Fig. 4A), responses are strongest when either tone is near BF (positions 1 and 5); they diminish considerably when the BF is midway between the tones (position 3), suggesting relatively good spatial separation between the representations of each tone. When the tones are closely spaced (ΔF = .25 octave; left panel of Fig. 4A), the responses remain relatively strong at all positions, suggesting that the representations of the two tones are not well separated. More importantly, the average rate profiles are similar for all presentation modes: in all cases the responses are well-segregated with significant dips when the tones are far apart (ΔF = 1 octave), and poorly separated (no dips) when the tones are closely-spaced (ΔF = 0.25 octaves). Thus, based on average rate responses, the neural data mimic the perception of the asynchronous but not the synchronous tone sequences. Therefore, the distribution of average rate responses does not appear to represent a general neural correlate of auditory streaming.

Figure 4
Responses of single-units to alternating (non-overlapping and partially-overlapping) and synchronous two-tone sequences at three different intervals (ΔF = 0.25, 0.5, 1 octaves). The two-tones were shifted relative to the BF of the cell in five ...

Instead of averaging the responses from all cells, we tabulated the number of cells indicating a significant segregation in the responses (implying a percept of 2 streams) or no segregation (a percept of 1 stream) by examining whether a significant dip occurred in each cell’s profile during the two extreme presentation modes (synchronous versus alternating tones). The determination of a dip was derived for each condition by finding a significant difference (one-tailed t-test; P < 0.025) between the distributions of the maximum response at either of the “BF sites” (1 or 5) compared with the minimum response at any of the non-BF sites (2,3, or 4). For the purposes of this analysis, we used a population of 66 units for which positions 1 or 5 were “BF sites”, and measurements were completed at all positions (1–5). In most experiments, several units with diverse BFs were recorded simultaneously with multiple electrodes, and hence it was only possible to match the tone frequencies to the BF of one or two of the cells. The percentage of cells with a significant dip in their profiles is shown in the histograms of Fig. 4B. We also calculated the magnitude of the dip (see Method) for each unit, and established that there was no significant difference in neural responses between synchronous and alternating modes (two-tailed t-test, P = 0.54 at 0.25 octave, P = 0.37 at 0.5 octave, and P = 0.42 at 1 octave), and that spatial segregation increases significantly with increasing ΔF (one-tailed t-test, shown in Figure 4B). The results show that (1) segregation is strongest at 1 octave separation, and weakest at 0.25 octaves, and that (2) there is little difference between the patterns of responses to the synchronous and alternating sequences. Thus, this alternative individual-cell response measure also fails to predict the different streaming percepts of the alternating and synchronous tones.

Experiment II: Frequency range of interactions

The key question of interest in this experiment was whether the range of interactions between the two tones was significantly different in the three presentation modes (alternating, overlapping, or synchronous). We measured the frequency range of interactions between the two tones by fixing tone A at the BF of the isolated unit, while placing tone B at ±1/3, ±2/3, ±1, ±1.5, and ±2 octaves around the BF (Fig. 3B). We also estimated the unit’s frequency tuning by measuring the iso-intensity response curve with a single tone sequence (black curve in Figure 5A). Other methodological details can be found in the Methods.

Figure 5
Averaged responses from a total of 64 units tested for alternating, synchronous and overlapping (tested in only 41/64 units) sequences using the paradigm of Experiment II. (A) The tuning near the BF averaged from all units. The average iso-intensity response ...

The average spike counts are shown in Fig. 5A from a population of 64 single units (in the synchronous and alternating modes) and 41 units (overlapping mode) that were recorded separately from Experiment I. All data were combined by computing the iso-intensity response curve of each unit, centering it around the BF of the unit, and normalizing it by the response of the unit to the single BF tone. We then kept only the half of the tuning curve above or below the BF from which the full 2 octave range was tested. Such (half-tuning) curves from all units were then averaged for each condition. The results highlight the interactions observed as the tones approached each other in frequency. For instance, when tone B was far from tone A at BF (e.g., at ± 2 octaves), the effects of the B tone on the cell are relatively small and the firing rate in all modes was similar to that of the single tone at BF (the normalized rate of 1, indicated by the dashed line). As tone B approached BF, the responses become modulated, first decreasing and then increasing steeply beyond about 1 octave on either side of the BF. Apart from differences in absolute firing rates, the pattern of interactions was similar in all three presentation modes. For example, the frequency separations at which significant interactions ensue are similar, implying that the units’ receptive fields (or their tuning curves) are similar whether they are driven by synchronous, alternating, or partially overlapping sequences.

To further quantify the population responses, we computed the effective bandwidth of interactions for each unit, defined as the furthest frequency on either side of the BF at which response interactions between the two tones were significant (see Methods). The data from all units in the synchronous and alternating (non-overlapping) modes are displayed in the histogram of the differences between the two measured ranges in Fig. 5B. The scatter is mostly symmetric, with a mean not significantly different from zero (two-tailed t-test, P = 1). Hence, the bandwidth differences for individual units fail once more to account for the different streaming percepts evoked by the alternating and synchronous presentation modes. Similar comparisons were also performed for the overlapping vs. synchronous and overlapping vs. alternating modes. The bandwidth differences in both cases were also mostly symmetric, with a mean not significantly different from zero.


The results from the two physiological experiments in awake ferrets contradict the hypothesis that segregation of AI responses to two-tone sequences is sufficient to predict their perceptual streaming. Instead, our findings reveal that synchronous and nonsynchronous sequences do not differ appreciably in the spatial representations of their temporally-averaged responses in AI, despite the substantial differences in their streaming percepts. Clearly a model that is successfully able to predict perception from these neural data will need to incorporate the time dimension.

C. Computational Modeling

Model structure

Based on our physiological and psychophysical results, we propose a new model of the relationship between cortical responses and auditory streaming, which can account for the finding that synchronous and non-synchronous tone sequences evoke very different percepts.

The model is based on the premise that temporal coherence between different sound features (e.g., frequency) is a fundamental organizing principle underlying the formation of perceptual streams. Specifically, auditory channels whose activity is positively correlated over time are assigned to the same perceptual stream, whereas channels with un- or anti-correlated activation patterns are assigned to different streams. In this way, the model combines in a general framework aspects of sequential and simultaneous auditory grouping, which in the past have often been treated as separate areas of research (e.g., Darwin and Carlyon, 1995b).

The model consists of two stages, which are schematically depicted in Fig. 6A. The first stage (temporal integration) takes as input an auditory spectrogram of a physical stimulus. The signal in each frequency band or “channel” of this spectrogram is passed through an array of bandpass filters tuned to frequencies between 2 to 32 Hz (see Methods for details); these “rate filters” perform temporal integration with time-constants ranging from 50 to 500 ms, consistent with the multi-scale dynamics of cortical responses (Chi et al., 1999). In the second stage (coherence analysis), a windowed-correlation between each pair of channels is computed by multiplying the outputs from filters corresponding to different channels with each other. The result is represented as a “dynamic coherence matrix” (denoted C), i.e., a correlation matrix that evolves over time. Effectively, the model computes the coincidence between all pairs of channels viewed over a range of time-scales of the order of tens to hundreds of milliseconds, consistent with experimentally observed cortical temporal responses (Kowalski et al., 1996a, b; Miller et al., 2002). Cortical responses typically phase-lock only to relatively slow temporal modulations of less than 30 Hz (Miller et al., 2002). Consequently, measuring correlations between cortical responses must be commensurate with these time-scales, allowing for a window long enough to include multiple periods of such responses.

Figure 6
Schematic of the coherence Analysis Model

Model predictions

Figure 6B shows simulated coincidence matrices corresponding to alternating (upper panel) and synchronous (lower panel) tone sequences (depicted in Fig. 6B, left). The right panels of Fig. 6B represent dynamic coherence matrices averaged over time, and capture both the average spatial distribution of activity as well as temporal coherence within and across channels. The diagonal entries merely reflect the average power in the input channels, and are not predictive of the perceptual organization of the sequences. The off-diagonal entries are indicative of the correlation (or lack thereof) across different channels, and are predictive of how the stimulus sequences are perceptually organized: these entries are at zero for the alternating sequence, because in this case the activation patterns in the channels corresponding to the A and B tones are out-of-phase (i.e., anti-correlated). In contrast, for the synchronous sequence, the off-diagonal entries corresponding to channels 1 and 5 are non-zero, reflecting the fact that these channels are activated in a coherent way (i.e., in phase).

To quantify the difference between these matrices, we used an eigenvalue analysis in order to decompose each matrix its maximally coherent components (Golub and Van Loan, 1996). Intuitively, this spectral decomposition of the coherence matrix allows us to determine which channels are positively correlated with each other (hence possibly forming one stream), and anti-correlated with different channels (which would form a separate stream). By performing an eigen-decomposition of the coherence matrix, we are effectively determining the number of axes (or independent dimensions) that best capture the data and, by analogy, the number of streams present in the stimulus. Hence, the rank of the matrix (or number of independent dimensions) can be interpreted as the number of auditory streams into which the stimulus sequence was likely to be perceptually organized by human listeners. Thus, a matrix of rank 1 (i.e., a matrix that can be fully decomposed using a single eigenvalue) is interpreted as reflecting a single perceived stream, a matrix of rank 2 is associated with a percept of two streams.

Using this model, we can relate the perceptual organization of the synchronous and alternating sequences to the neural responses of the cortical units obtained in neurophysiological Experiment I. The responses for each stimulus position (numbered 1 through 5) are equivalent to responses from different cortical sites, corresponding to five different “channels” along the spectral axis. We accumulated the peri-stimulus time histograms (PSTHs) for each position and each stimulus condition by averaging over the 10 presentation times at a resolution of 1 msec. These 5-channel histograms for each stimulus condition (presentation mode and ΔF) are then given as input to the temporal integration and coherence analysis model, in order to derive coherence matrices similar to those described in Fig. 7.

Figure 7
Coherence Analysis from neural population

Figure 7A shows the mean coherence matrices (across all 66 neurons in the recorded BF-sites sample from Experiment I) for alternating (top row) and synchronous (bottom row) tone sequences with frequency separations (ΔFs) of 0.25 (left), 0.5 (middle), and 1 (right) octave. As explained earlier, the positive diagonal entries reflect overall activity in each frequency channel. These positive diagonal entries show decreasing activity in the intermediate channels (2–3) with increasing frequency separation between the tones, reflecting increasing tonotopic separation in the measured cortical activation patterns. The fact that this pattern is observed for both synchronous and alternating tones confirms that tonotopic separation per se is therefore not a valid indicator of the perceptual organization of these stimulus sequences. In contrast, the activation of the off-diagonal entries in these coherence matrices follows a pattern that is more closely aligned to perception, with more activation found in conditions that are perceived as a single stream.

The predicted number of streams, as determined by the number of “significant” eigenvalues (see Methods) is shown in the upper left corner of each panel in Figure 7. For the synchronous conditions, the coherence matrices always yielded a single significant singular value, even at the largest two ΔFs (0.5 and 1 octave) (singular values for each matrix are included in the figure caption), in line with the perception of a single stream. In contrast, in the alternating conditions, a second significant eigenvalue was observed at frequency separations of 0.5 and 1 octave, in line with the perception of two streams

The model presented here can be adapted so as not to rely only on the rank of the coherence matrix to predict the perceptual organization from the input. The size of the eigenvalues, as well as the shape of the eigenvectors, is also a strong indicator of the different dimensions (or streams) in the scene. To illustrate this claim, we performed a simulation using sequences of two tones (A and B). The low tone was fixed at 300 Hz, while the high note was fixed at 952 Hz. Both tones were 75 ms long. The onset delay between the A and B tones was varied from 0 ms (ΔT=0%) to fully alternating (ΔT=100%). Figure 8A shows the coherence analysis for the latter case, and reveals that the coherence matrix has rank 2 (indicating a 2-stream percept). The ratio of the second-to-first singular values (λ21) equals 0.93 indicating that both λ-values are almost equal. In contrast, Figure 8C shows the case of complete synchrony; and reveals that the coherence matrix can be mapped on 1 main dimension, hence correlating with the percept of 1 stream. In this case, the ratio λ21 is equal to 0.01, revealing that the second singular value is close to zero. Using the relative sizes of the first and second singular values (not just the rank of the matrix), we can explore the ‘strength’ or ‘confidence’ of the percept of one or two streams as we vary the degree of asynchrony. Figure 8B shows the decrease in this ratio as ΔT is gradually varied from 100% to 0%, hence allowing us to parametrically follow the influence of degree of asynchrony on grouping of two frequency streams, thereby allowing us to predict the transition between the percepts of one and two streams.

Figure 8
Simulation of two-tone sequences with varying asynchrony

General Discussion and Conclusions

Evidence against a purely tonotopic or “spatial” model of auditory streaming

We examined the hypothesis that acoustic stimuli exciting spatially segregated neural response patterns are necessarily perceived as belonging to different perceptual streams. This “spatial” hypothesis underlies (explicitly or implicitly) previous interpretations of the neural correlates of streaming in the physiological investigations and the computational models of streaming (Beauvois and Meddis, 1991, 1996; Bee and Klump, 2004, 2005; Fishman et al., 2004; Fishman et al., 2001; Hartmann and Johnson, 1991; Kanwal et al., 2003; McCabe and Denham, 1997; Micheyl et al., 2007b; Micheyl et al., 2005; Pressnitzer et al., 2008). One of the elegant aspects of the “spatial” hypothesis is that it can be generalized to predict that separate streams will be perceived whenever sounds evoke segregated responses along any of the representational dimensions in the auditory cortex, including not just the tonotopic axis, but also a fundamental-frequency (F0) or “virtual pitch” axis (Bendor and Wang, 2005, 2006; Gutschalk et al., 2007), as well as, perhaps, temporal- and spectral-modulation-rate axes (Bendor and Wang, 2007; Kowalski et al., 1996a, b; Schreiner, 1998; Schreiner and Sutter, 1992; Sutter, 2005; Versnel et al., 1995), thereby accounting for psychophysical findings of stream segregation induced by differences in F0 or modulation rate in the absence of tonotopic cues (Grimault et al., 2002; Roberts et al., 2002; Vliegen and Oxenham, 1999).

However, the experimental data reported here cast doubt on the validity of an explanation of auditory streaming in terms of neural-response separation that ignores temporal coherence as an important determinant of perceived segregation. Our human psychophysical results show very different perceptual organization of synchronous and asynchronous tone sequences, whereas the extent of segregation of the neural responses in ferret AI was essentially independent of the temporal relationships within the sequences. This finding emphasizes the fundamental importance of the temporal dimension in the perceptual organization of sound, and reveals that tonotopic neural-response separation in auditory cortex alone cannot explain auditory streaming.

A spatio-temporal model of auditory streaming

Our alternative explanation augments the spatial (tonotopic) segregation hypothesis with a temporal dimension. It is a spatiotemporal view, wherein auditory stream segregation requires both separation into neural channels and temporal incoherence (or anti-coherence) between the responses of these channels. This spatiotemporal hypothesis predicts that if the evoked neural responses are temporally coherent, a single stream is perceived, regardless of the spatial distribution of the responses. This prediction is consistent with our psychophysical findings using synchronous tone sequences. The prediction is also consistent with the introspective observation, confirmed in psychophysical studies, that synchronous spectral components generally fuse perceptually into a single coherent sound (e.g., a vowel or a musical chord), whereas the introduction of an asynchrony between one and the other components in a complex tone results in this component “popping out” perceptually (Ciocca and Darwin, 1993).

The present demonstration of a critical role of temporal coherence in the formation of auditory streams does not negate the role of spatial (tonotopic) separation as a factor in stream segregation. The extent to which neurons can signal temporal incoherence across frequency is determined in large part by their frequency selectivity. For example, the responses of two neurons tuned to the A and B tones in an alternating sequence (Fig. 1A) can only show anti-coherence if the frequency-selectivity of the neurons is relatively high compared to the A-B frequency separation. If the neurons’ frequency tuning is broader than the frequency separation, both neurons are excited by both tones (A and B), and respond in a temporally coherent fashion. In this sense, spatial separation of neural responses along the tonotopic axis may be necessary for stream segregation but, as this study shows, it is not sufficient.

The principle of channel coherence can be easily extended beyond the current stimuli (see Supplementary material for further simulations) and the tonotopic frequency axis to include other auditory organizational dimensions such as spectral shape, temporal modulations as well as binaural cues. Irrespective of the nature of the dimension explored, it is the temporal coherence between the responses along that dimension that determines the degree of their integration within one stream, or segregation into different streams.

Finally, there are interesting parallels between the present findings, which suggest an important role of temporal coherence across sensory channels in auditory scene analysis, and findings in other sensory modalities such as vision, where grouping based on coherence of temporal structure has been found to provide an elegant solution to the binding problem (e.g., (Alais et al., 1998; Blake and Lee, 2005; Fahle, 1993; Treisman, 1999). Together, these findings suggest that although the perceptual analysis of visual and auditory “scenes” pose (at least, superficially) very different problems, they may in fact be governed by common overarching principles. In this regard, parallels can be drawn between prominent characteristics of auditory stream formation, such as the buildup of streaming and its dependence on frequency separation, and processes involved in the visual perception of complex scenes.

Do percepts of auditory streams emerge in or beyond primary auditory cortex?

For neural activity in AI to be consistent with the psychophysical observation that synchronous tones with remote frequencies are grouped perceptually while alternating tones are not, there should be cells in AI whose output is strongly influenced by temporal coherence across distant frequencies. While such cells are likely to be present in AI (Barbour and Wang, 2002; Kowalski et al., 1996b; Nelken et al., 1999), we did not systematically find many that reliably exhibited the properties necessary to perform the coincidence operation. For example, all neurons sampled in this study followed the temporal course of the stimuli (with increased firing rates during epochs where at least one tone was present), the responses did not unambiguously increase in the presence of temporal coherence across tonotopic channels. Therefore, one possibility is that the percepts of stream segregation and stream integration are not determined in AI. Another possibility is that the coincidence and subsequent matrix decomposition described in the model are realized in a different, less explicit, form. For instance, it is theoretically possible to replace the spectral decomposition of the coherence matrix by a singular-value-decomposition directly upon the arrayed cortical responses. The spectral decomposition of the coherence matrix is equivalent to PCA analysis of the covariance matrix of the channel responses. Equivalent results can be computed by a singular value decomposition directly on the channel temporal responses, i.e., without computing the covariance matrix, obviating the need for the coincidence detectors. This leaves open the question of how and where, in or beyond AI, the detection of temporal coincidences across remote frequency channels is neurally implemented (Nelken, 2004).

The auditory streaming paradigm, with its relatively simple and well-controlled stimuli and extensively characterized percepts, may in fact provide an excellent vehicle to explore a broader issue in brain function: that of the relationship between perception and neural oscillations, which reflects coherent responses across different regions in the brain. Coherence as an organizing principle of brain function has gained prominence in recent years with the demonstration that it could potentially play a role in mediating attention (Liang et al., 2003; Zeitler et al., 2006), in binding multimodal sensory features and responses (Lakatos et al., 2005; Schroeder et al., 2008), and in giving rise to conscious experiences (Fries et al., 1997; Gross et al., 2007; Meador et al., 2002; Melloni et al., 2007). Our results reinforce these ideas by emphasizing the importance of temporal coherence in explaining auditory perception. Specifically, the inclusion of the time dimension provides a general account of auditory perceptual organization that can in principle deal with any arbitrary combinations of sounds of time and frequency.

Attention and the neural correlates of streaming

Interpretations of neural responses recorded in passive animals as “correlates” of auditory percepts are necessarily speculative, since behavioral measures of the animal’s percepts during the recordings are not available. Under such conditions, the experimenter can, at best, assert that the neural responses differ across experimental conditions (e.g., different stimuli) in a way that is consistent with behavioral measurements obtained in the same (or a different) animal (or species) under similar stimulus conditions. In this respect, the present study suffers from the same limitation as previous investigations of the neural basis of auditory streaming in awake animals that were either passive (Bee and Klump, 2004, 2005; Fishman et al., 2004; Fishman et al., 2001; Kanwal et al., 2003), or engaged in a task unrelated to streaming (Micheyl et al., 2005).

The possibility remains that AI responses to alternating and synchronous tone sequences in awake animals that are engaged in a task, which requires actively attending to the stimuli, might be substantially different from those recorded in passive animals. It is known that neural responses in AI are under attentional control, and can change rapidly as the task changes (Fritz et al., 2005a; Fritz et al., 2003; Fritz et al., 2005b). . Such attentionally driven changes in receptive fields might differentially affect the neural responses to alternating tones and those to synchronous tones, in a way that makes these responses more consistent with the percepts evoked by those sequences (Yin et al., 2007). On the other hand, the aspects of streaming investigated here – in particular the increased segregation with increasing frequency separation in asynchronous conditions – have been posited to be “automatic” or “primitive” and hence independent of attention (Macken et al., 2003; Sussman et al., 2007), although the matter is still debated (Carlyon et al., 2001).

The possible effects of attention could be investigated in future studies by controlling the attentional and behavioral state of the animal. Our model postulates the existence of units that should exhibit a dependence on temporal coherence. We have not found such units in AI, and therefore a future search may concentrate more fruitfully on other, supramodal, areas, such as the prefrontal cortex, where attentional modulation of AI responses may originate (Miller and Cohen, 2001).




Nine listeners took part in the study (none of authors participated in these tests). They all had normal hearing (defined as pure-tone hearing thresholds less than 20 dB HL at octave frequencies between 250 and 8000 Hz) and extensive experience with the test procedure and stimuli.


The stimuli were sequences of A and B tones, where A and B represent different frequencies. The frequency of the A tone was kept constant at 1000 Hz. The frequency of the B tone was set 0.5, 0.75, or 1.25 octaves above that of the A tone. Each tone was 100 ms in duration, including 10-ms raised-cosine onset and offset ramps. Each sequence consisted of five “precursor” tones at each frequency (i.e., five A tones and five B tones), followed by two “target” tones (i.e., one A tone and one B tone). Depending on the condition being tested, the precursor A and B tones were either synchronous or asynchronous. In the synchronous case, all the tones were separated by silent gaps of 50 ms; in the asynchronous case, the gap between consecutive precursor B tones was still 50 ms, but the gap between consecutive A tones was either 30 ms or 70 ms, depending on the condition being tested. Depending on the observation interval, the target B tone was either separated from the preceding precursor B tone by the same 50-ms silent gap as consecutive precursor B tones (standard interval), or it was shifted forward or backward in time by an amount, ΔT, which was controlled by the adaptive threshold-tracking procedure (signal interval). The two parallel sequences of A and B tones in each interval were always positioned in time relative to each other in such a way that the target A and B tones were synchronous in the standard interval, and were shifted by ΔT in the target interval. In addition, a control condition was run, in which the A tones were turned off, and the B tones were generated in exactly the same way as described above.


Thresholds were measured using a two-interval, two-alternative forced-choice (2I-2AFC) procedure with an adaptive three-down one-up rule, which tracked the 79.4%-correct point on the psychometric function. On each trial, two sequences were presented, separated by a silent gap of 500 ms. In one of those sequences (the standard interval), the target A and B tones were synchronous; in the other (the target interval), they were asynchronous. The order of presentation of the two sequences was randomized, with each sequence being a priori as likely as the other to come first. The listener was informed of this fact, and asked to indicate after each trial, which of the two sequences (first or second) contained the asynchronous A and B tones at the end. Listeners gave responses by pressing keys (“1” or “2”) on a computer keyboard. At the beginning of each adaptive run, the tracking variable, ΔT, was set to 20 ms. It was divided by a factor c after two consecutive correct responses, and multiplied by that same factor after each incorrect response. The value of c was set to 4 at the beginning of the adaptive run; it was reduced to 2 after the first reversal in the direction of tracking (from decreasing to increasing), and to √2 after a further two reversals. The procedure stopped after the sixth reversal with the √2 step size. Threshold was computed as the geometric mean of ΔT at the last six reversal points. Each listener completed at least four threshold measurements in each condition. The psychophysical data shown in this article are geometric mean thresholds across listeners.


The stimuli were generated digitally and played out via a soundcard (LynxStudio L22) with 24-bit resolution and a sampling frequency of 32 kHz, and presented to the listener via the left earpiece of Sennheiser HD 580 headphones. Listeners were seated in a double-walled sound-attenuating chamber (Industrial Acoustics Company).


Experimental design

The stimuli were sequences of A and B tones, where A and B represent different frequencies as illustrated in Figure 1. Both alternating (non-overlapping and partially-overlapping) and synchronous sequences were used (see details below). In Experiment I, Tones A and B were shifted equally in five steps relative to a unit’s BF as shown in Figure 3A, with tone B starting at the BF and tone A ending at the BF. ΔF between the tones was 0.25, or 0.5, or 1 octave, which was fixed within a trial and varied among different trials. The total number of conditions was 45 (5 positions × 3 ΔF × 3 modes). In Experiment II, tone A was set at the BF of the isolated unit, while tone B was placed to ±1/3, ±2/3, ±1, ±1.5, and ±2 octaves away from tone A if applicable, as illustrated in Figure 3B. The stimuli also included a single tone sequence to measure the frequency tuning of the unit.

In both experiments I and II, each trial included 0.4 sec pre-stimulus silence, 3 sec stimulus length, and 0.6 sec post-stimulus silence. Tone duration was 0.075 sec including 0.005 sec onset and offset ramps, and an inter-tone gap of 0.025 sec in the alternating sequence and 0.125 sec in the synchronous sequence. For the overlapped sequences, the tone onset asynchrony was 40 ms (i.e., 50% overlap between the tones). All conditions were presented pseudo-randomly 10 times at 70 dB or at about 10 dB above threshold of the isolated units.

Data analysis

For each unit and each condition, period histogram was constructed from the peri-stimulus time histograms (PSTH) by folded (averaged) responses to the two tones over the duration of the trial from 0.6 to 3 sec after the onset of the stimuli. Examples of such response histograms from a single unit the stimuli of Experiments I are shown in Supplementary Figures 1 and 2. For each stimulus response, we excluded the first 0.6 sec so as to avoid adaptation effects. The mean firing rate (spikes/sec) was computed by taking the average value of the period histogram (averaged over 0.2 sec). The overall firing rate patterns were obtained by averaging the normalized responses from all isolated units. In order to compensate for inherent differences in the relative strength of tone responses across units, firing rates were first normalized by dividing them by the maximum rate at each ΔF and at each stimulus mode in experiment I and by the mean firing rate at BF in experiment II.

The magnitude of dip was determined according to the following equation:


where ‘Side’ is the maximum response at either of the “BF sites” (position 1 or 5); and ‘Center’ is the minimum response at any of the non-BF sites (positions 2, 3 or 4).

To measure the effective bandwidth of interaction between tones, the mean firing rate at the frequency closest to BF (i.e., 1/3 or −1/3 octave) was compared with those at the other frequencies on the same direction (i.e., below BF or above BF). The frequency showed the significant difference (two-tailed t-test, P < 0.05) in mean firing rate from the frequency closest to BF was the effective bandwidth of interaction.


The neural responses to the shifting two-tones (Experiment I) from N=66 (BF-sites) neurons are pooled together and processed through a ‘coherence analysis’ as follows.

A per-stimulus time histogram (PSTH) is constructed for each stimulus condition (i.e. a given tone synchrony configuration, a specific frequency separation and a position relative to the spectral response of the neuron), by averaging the responses across 10 stimulation trials using 1 msec bins. Each PSTH sequence is then convolved in time with an array of filters, with impulse response hΨ(t), parameterized by Ψ =(ωc, θc), and defined as:




and g(.) and ĝ(.) denote Hilbert transform pairs (Bracewell, 1999). Each filter hΨ(t) is characterized by two tuning parameters: ωc, a characteristic rate which varies over the range [2 4 8 16 32] Hz; and θc, a characteristic phase, which is set to span the entire range [0,2π] in steps of π/3. The characteristic rates are chosen to cover the range of temporal modulations observed in tuning properties of cortical neurons, while the phases vary to allow different phase configurations of the impulse response of the rate analysis. The seed function g(.) is chosen to be a gamma function (as shown in the inset of the model schematic in Figure 6A) (Chi et al., 1999). Functionally, this temporal filtering stage integrates the temporal response from each channel over a range of analyses windows, yielding a 3 dimensional representation covering time (t), channel (i) and rate (Ψ).

The responses from all channels are pooled together in a vector representation,


where [.]’ denotes the transpose operator. These responses are the cross-correlated with each other (via an inner product operation), and averaged across the entire array of rate filters to yield a cross-correlation matrix C(t), whose entries are defined as:


The matrix C captures the degree of coherence in the neural responses at different frequency locations along the tonotopic axis. A high correlation value between two channels indicates a strong coherent activity at these two locations, while a low correlation value indicates lack of coherent neural activity.

To estimate the baseline level for average eigenvalue level in case of random coherence, we simulate activity of 66 different neurons with random PSTH activity over 3 seconds duration for 5 different positions. These random PSTHs are then processed through the coherence analysis, yielding a matrix C of random correlations among channels. The matrix from each ‘random’ unit is first normalized to unit-norm, before averaging across all units to yield one ‘random’ coherence matrix. The SVD decomposition of this final matrix produces a full rank matrix, with five almost equal eigenvalues.

Supplementary Material



Download audio file.(77K, mp3)


Download audio file.(77K, mp3)


Download audio file.(77K, mp3)


This work was supported by grants R01 DC 07657 and CRCNS R01 AG02757301 from the National Institutes of Health. We thank Cynthia Hunter for assistance in collecting the psychophysical data, and Pingbo Yin and Stephen David for assistance with physiological recordings. We also thank three anonymous reviewers for their valuable comments.


Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errorsmaybe discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.


Supplemental Data include two figures and three audio files and can be found with this article online at http://www.neuron.org/supplemental/S0896-6273(08)01053-2


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